87 190D 2.5, rear window doesn't close.

today, just one hour before a thunder storm, my driver side rear window stopped functioning. It did this once before when it refused to open, but
this time it stays wide open.
I tried both the switch on the console and that on the door itself, but neither works. Has anyone experienced this? Any idea where I should look the first? Could it be a fuse? or cable problem?
Many thanks for any hint or tip.
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